[Amps] SB-220 amp help

[Pete Smith]

My SB-220 has been an absolute joy since I bought it a decade ago, at 
which point it was already about 25 years old, but this message raised a 
question.  In CW contest service, I have always used the SSB position, 
and it puts out >1200 watts on every band but 10M.  No major component 
failures, though it did melt the solder in the 3-500 filament pins until 
I swapped the socket clips and polished the pins to improve contact.

Only one issue.  For some reason, the high voltage and relative power 
positions on the multimeter quit years ago, while the grid current still 
works.  I didn't really need them, but it would be nice to have 
everything working.  I'm assuming a common cause, but I'll be darned if 
I can see a component or components that would cause this, other than 
the switch itself.  Can anyone point me in the right direction?

73, Pete N4ZR

The World Contest Station Database, updated daily at www.conteststations.com
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On 4/6/2011 8:14 AM, Gudguyham at aol.com wrote:
> GM guys, since we are beating the hell out of the Heathkit amplifier these
> past few days, I want to offer some help to you who are new to the amp.
> Here is an occasional problem that comes up that can frustrate you.  Does
> your sensitivity meter not work right?  Does it only give you a reading a  one
> point is its rotation?  Then when it does, the meter slams against the
> pin?  SO you say oops, my sensitivity pot is bad, and you replace it and as
> soon as you put power through your new pot it goes south again?  The  problem
> is leakage in the 1N191 detector diode. You need to change the  diode or
> else you will burn up any number of pots you replace.  The 1N191  is not made
> anymore and it looks like 1 1N60 is a good replacement.  This  tip might save
> you a lot of aggravation.  I am assuming what happens is  that the diode
> leaks RF and it burns the pot as I have dissected several pots  and found the
> same "burn" on all that were bad. Happy  motoring.
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